Wilpinjong CHPP Expansion Project - Mudgee, NSW

Project featuring the RECo TerraPlus® System.

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General Information

The Wilpinjong Mine is an open-cut mine located 40 kilometres north east of Mudgee that produces a high quality thermal coal for export and domestic markets. The Wilpinjong expansion project featured a collaborative project delivery approach between Peabody and Ausenco Taggart offices located in Australia, the United States and China. Utilising a not-to-exceed reimbursable contract model, Chinese delivered fabrication, US based drafting and a local Project Management and Engineering team proved to maximise value for the Project Owner.

The project scope of work included the expansion of the existing 66kV substation and high voltage reticulation to the CHPP expansion. A secondary ROM system featuring a 400 tonne ROM bin, feeder breaker, scalping screen, secondary and tertiary stacker deliver raw coal to a stockpile dedicated to the CPP, while the existing sizing system is free to dedicate to direct shipped coal.

The addition of a fine dense medium circuit has increased the plant capacity from 800t/h to 1,200t/h, and the addition of a 100,000 tonne washed coal stockpile improves storage and flexibility for product shipments

This work was the second subcontract to be carried out at Wilpinjong by Retaining Solutions. The first ROM wall was completed in 2006. This package involved a greater scope involving the full turnkey solution from foundation design through to the complete civil earthworks package and rewall construction up to underside of dump slab. The Reinforced Earth Company was engaged for the design and supply of the 16.8m high reinforced earth TerraPlus® panel wall. A detailed summary of the project is described herein.

As with any retaining wall design the most important information is that of the adequacy of the foundation below the footprint of the proposed structure for both drained and undrained conditions. With limited geotechnical information the potential for latent conditions is always a high risk contractually. This particular application being a reinforced earth structure 16.8m high with a 320tonne dump truck tipping off the edge of a 1m thick slab requiring foundation allowable bearing capacities in the order of 550-720kPa. There was limited geotechnical information in the vicinity of the proposed rewall with only 1No borehole some 20m away from the actual wall location. As a result further in-situ methods were adopted in order to confirm suitable foundation design models. Test pits were carried out with a 30t excavator with visual inspection by an experienced geotechnical engineer.

The Site

When making an assessment of the subsurface conditions across a site from a limited number of test locations it should be recognised that unforeseen variations may occur between these locations. The data derived from all aspects of the site investigation program are extrapolated across the site to form a geotechnical model and then an engineering opinion is provided on overall subsurface conditions and their likely behaviour with the proposed development. The actual conditions could very possibly differ from those inferred herein, especially in a geotechnical environment such as this which has significant deposits of alluvial/colluvial soils and where there have been previous mining activities. No subsurface exploration program, no matter how comprehensive, can reveal all subsurface details and anomalies.

As previously mentioned the only borehole located some 20m away from the excavation indicated that there was 2.5m of uncontrolled fill over 2.5 m of firm clay/silt, which was underlain by rock at 5 m depth below existing surface level.

The proposed site for the new RE dump wall was previously covered by mine waste filling of significant depth. This fill extended for a couple of hundred meters to the west where coal was regularly stockpiled. There was a large water supply dam located some 250 m south west of the wall. In December 2010 the water level in this dam was at approx RL 412, some 6 m above the proposed wall toe level. Also there is a small sedimentation pond

approximately 100 m south of the wall. The water level in this pond appears to be at about RL 407. In the vicinity of the proposed new wall footprint the existing fill had been excavated down to RL 407, that is 1 m above the proposed wall toe level. There were significant ground water flows observed coming out of the ground, particularly from the western and southern sides of the excavation. Also the water appeared to be flowing up from the ground, possibly under artesian pressure. It was difficult to operate heavy machinery in the floor of the excavation due to the ground water inflows. The 30 tonne excavator sitting on a 1m high fill platform was used to dig a small test probe close to the centre of the proposed wall. The pit was excavated through water. There appeared to be some 1 m of further fill over a soft to firm natural inset silty clay/clayey silt. The pit was extended to 6 m depth, the maximum reach of the excavator. No in-situ rock was encountered. It is reported that since then further testing pitting has suggested that rock will be encountered at approximately 4 m depth below the underside of the wall.

There are three potential sources for these flows: from the upper dam some 250 m to the south west, from the lower sedimentation pond some 100 m to the south, or due to high pressure artesian flows emanating from the underlying rock profile.

In summary the various fieldwork indicated that the proposed walls were underlain by uncontrolled fill and weak in-situ soils overlying in-situ medium strength sandstone. The rock was approx 4 m below the underside of the RE block. The groundwater level was about 1 m above the underside of the RE block. The presence of significantly high ground water flows could not have been anticipated prior to commencing construction. These flows were unusual and had ramifications on both the design of the RE wall and the construction of an adequate foundation.

From the in-situ investigations carried out the most cost effective foundation improvement solution was to found the walls on the underlying sandstone using a raft of “new rock fill” (graded 150mm to 400mm max size cobbles/boulders). The depth of the new rock foundation fill raft was up to maximum depth of 4m below rewall footing level.

Key characteristics of the “Foundation Rock Fill Raft” solution were:

- Negligible differential settlement;

- High strength durable rock unlikely to breakdown when permanently submerged in water;

- Ability of material to pack down naturally during placement operations with negligible compaction effort during dry conditions or inundation.

Bidim A64 non-woven heavy geotextile, 2 layers, were placed above the rock fill foundations to ensure that the overlying RSW backfill and general fill did not ingress over time into the foundation zone. The geotextile was also extended up the sides of the excavation above ingress level at the rear of excavation. Chimney drains were adopted up to within 1.5m of dump slab level. Global stability of new designed structure was assessed using the STARES programme producing minimum factors of safety of 1.52 using Bishops method.

Retaining Solutions/Hardy Bros Mining were solely responsible for conditioning / loading / cartage / placement / spreading and compaction of all different materials to be placed beneath, within and to the rear of the reinforced earth structure. All backfilling operations were carried out under strict backfilling and compactions specifications. Daily Quality verification Checklists were

A successful part of this works package with Hardy Bros Mining was the management of materials handling operations within a 1.3km haul route from rewall location. Such haul routes dictated close interaction with active mine haul roads controlled by Thiess Mining. Haul Route Traffic Management Plans and Risk Assessment were critical to ensure safe work practices and plant interaction.